The goal of this proposal is to develop a novel imaging detector for positron emission imaging. A basic detector module will consist of an array of parallel-piped LSO scintillators coupled to a size-matched silicon photodetector array whose elements provide superior energy and timing resolution. The photodetector will be developed using a unique new technology, namely: three-dimensional silicon processing. This approach will lead to lower capacitance, higher quantum efficiency and faster charge carrier collection compared with other solid-state detectors. Using this technology we plan to develop 2 x 2 cm2 (8 x 8 pixel) imaging modules utilizing pairs of LSO modules coupled to the new three- dimensional photodetector (3DP) array modules. The new detector modules could eliminate the need for expensive photomultiplier tubes and allow for integration of the electronics with the detector, which will lead to lower cost PET systems. Initially the devices would be optimized for use in small animal PET imagers. Cancer research using small animals and PET imaging techniques has grown in importance in the last few years and is continuing to become a significant research tool for investigating the efficacy of new radiopharmaceuticals for cancer therapy. The proposed detector development program has specific goals divided into two technical phases. Phase I: Design optimized detector structure leading to the targeted performance for electronic noise, quantum efficiency, fast transit time of electrons; (2) Fabricate prototype eight-by- eight element 3DP array; (3) Evaluate detector parameters including capacitance, dark current, noise, quantum efficiency, coincidence resolving time, spectral response to various radionuclides. These goals will be used to gauge the feasibility of the projects long-term aims. Phase II: (1) Develop and construct a pair of optimized 8 x 8 pixel LSO/SDP detector modules with input electronics; (2) Develop housing, specialized processing and display electronics; and (3) Evaluate coincidence imaging using prototype detector system under simulated clinical conditions at the UCLA School of Medicine. PROPOSED COMMERCIAL APPLICATIONS: In the third phase we will commercialize single and multiple detector modules based upon the Phase 1 and Phase 2 efforts, for clinical use in small animal positron emission imaging. These new detectors will replace the expensive PMT detectors currently used in PET detector systems and will reduce the overall cost. The new 3D silicon photodetector requires only low cost silicon planar processing and opens up the opportunity for integration of the signal electronics with the detector. There are large needs and potential for commercial uses for these detectors as PMT replacements for PET.